During recent years, magnetic resonance tomography (MR tomography) has established itself as a significant imaging method in medicine. MR imaging systems which generate sectional views of an object to be examined, in particular a human body or body part, using nuclear magnetic resonances are known per se. In this type of activity, the body to be examined is introduced into a strong homogeneous static magnetic field, this being known as the main magnetic field, which effects an alignment of the nuclear spin of atomic nuclei within the body, in particular hydrogen atomic nuclei (protons) which are bound to water. Said nuclei are then excited into a precessional motion by means of high-frequency excitation pulses. After the end of a corresponding high-frequency (HF) excitation pulse, the atomic nuclei precess with a frequency (the so-called Larmor frequency) which depends on the strength of the main magnetic field, and then settle down after a predefined tissue-dependent relaxation time into the preferred direction which is predetermined by the main magnetic field. Using computational and/or metrological analysis of the integral high-frequency nuclear signals, a picture relating to a body layer can be generated from the spatial spin density or the distribution of the relaxation times. The assignment of the nuclear resonance signal, which can be seen as a result of the precessional motion, to the location of its occurrence is done by applying linear field gradients. For this, corresponding gradient fields are superimposed on the main magnetic field and controlled such that an excitation of the nuclei only occurs in a layer which is to be depicted. An HF coil apparatus is required both for the HF excitation of the nuclear spin and for the detection of the nuclear response signal. Imaging systems which are based on these physical effects are also referred to using the terms nuclear-spin tomography, nuclear magnetic resonance (NMR) tomography or magnetic resonance imaging (MRI).
Variations of the magnetic fields, in particular the spatial distribution of the magnetic field strengths, from their theoretically predetermined and calculated values cause geometric distortions in the picture. In particular, methods which are based on a quantitatively precise geometric representation of the areas to be examined are considerably impaired in respect of their quality as a result of such distortions.
It is possible primarily to identify four different causes which result in geometric distortion:
The magnetic susceptibility varies in the case of different materials and thus gives rise to a slight material-dependent modification of the main magnetic field.
Static magnetic field homogeneities are largely equalized by the so-called shimming of the magnetic field, but slight residual inhomogeneities can nonetheless remain.
Non-linearities of the gradient coils cause distortions in all directions. Gradient coils are usually designed in such a way as to minimize such distortions in the isocenter of the device. Consequently, such effects primarily occur in the marginal areas of MRT pictures.
Eddy currents are always produced in conductive materials when modifications of the magnetic fields are carried out. This primarily occurs when switching the gradient fields. In this case the strength and the spatial distribution of the eddy currents depend on the MRT sequence which is applied. In turn, the eddy currents themselves cause magnetic fields which overlie other magnetic fields and therefore cause geometric distortions.
One possibility for detecting geometric distortions which occur in the case of a specified MRT sequence is the use of a phantom. Such phantoms generally have a three-dimensional lattice of structures which are shown clearly in an MRT picture. Using an image of the phantom, the image of the three-dimensional lattice can be compared with the original lattice. This allows the creation of a distortion map which identifies the strength and the direction of the geometric distortions at various spatial points. This distortion map is used for correcting geometric distortions in the context of pictures which are subsequently recorded using the same MRT sequence.
Phantoms which are suitable for such a method generally exhibit structures which are easily and reliably identified, e.g. spheres, at the lattice points. An exact and reproducible specification of the position of the spheres, e.g. their midpoints, in an image of the phantom is necessary in this case in order to capture distortions.
Phantoms whose three-dimensional lattice is formed of spheres are disclosed in M. Breeuwer et al., “Detection and correction of geometric distortion in 3D MR images”, Proc. SPIE 4322, 1110-1120, 2001, and in M. Holden et al. “Sources and correction of higher order geometrical distortion for serial MR brain imaging”, Proc. SPIE 4322, 69-78, 2001. In these documents, as a midpoint of the spheres, only the central area of the sphere is specified in the image of a sphere, after the image of a sphere has been modified by a series of morphological erosion and dilatation operations to such an extent that additionally interfering structures—such as connections between the spheres—are removed from the image. However, the images of the spheres are also modified as a result of the erosion and dilatation operations, presenting the risk that the central area of the sphere will also be displaced in the case of spheres which are depicted in a distorted manner.
U.S. Pat. No. 5,545,995 A discloses a calibration method for correcting geometrical imaging errors, which are caused by gradient field non-linearities and by magnetic field inhomogeneities, in an MR image with reference to a three-dimensional scan of a phantom. The phantom contains an array of conical rods which generate an array of corresponding images in reconstructed picture layers. The size of the corresponding images and their position allow the measurement of positional errors and the creation of correction factors for subsequently acquired images of patients.
US 2005/0024051 A1 likewise discloses a method for correcting distorted pictures whose distortions are caused by non-linear gradient fields and translational, rotational, and/or winding or design errors of gradient coils. In this context, developments of surface spherical harmonics of the gradient fields and rapid Fourier transformation techniques are used in order to produce corrected pictures.
DE 101 07 421 A1 and the corresponding US 2002/0110268 A1 disclose in each case a method for detecting imaging distortions in an image. A first region of the imaging volume is shown free of distortion in the imaging, while a second region is distorted. The method works using at least three markings which show a known spatial position relative to each other and of which two markings are located in the first region of the imaging volume. Distortions in the image can be inferred from the position of the markings relative to each other.